Melanocortin-4 receptor mutations paradoxically reduce preference for palatable foods

The Melanocortin System: A Promising Target for the Development of New Antidepressant Drugs

Major depression is one of the most prevalent mental disorders, causing significant human suffering and socioeconomic loss. Since conventional antidepressants are not sufficiently effective, there is an urgent need to develop new antidepressant medications. Despite marked advances in the neurobiology of depression, the etiology and pathophysiology of this disease remain poorly understood. Classical and newer hypotheses of depression suggest that an imbalance of brain monoamines, dysregulation of the hypothalamic-pituitary-adrenal axis (HPAA) and immune system, or impaired hippocampal neurogenesis and neurotrophic factors pathways are cause of depression. It is assumed that conventional antidepressants improve these closely related disturbances. The purpose of this review was to discuss the possibility of affecting these disturbances by targeting the melanocortin system, which includes adrenocorticotropic hormone-activated receptors and their peptide ligands (melanocortins). The melanocortin system is involved in the regulation of various processes in the brain and periphery. Melanocortins, including peripherally administered non-corticotropic agonists, regulate HPAA activity, exhibit anti-inflammatory effects, stimulate the levels of neurotrophic factors, and enhance hippocampal neurogenesis and neurotransmission. Therefore, endogenous melanocortins and their analogs are able to complexly affect the functioning of those body’s systems that are closely related to depression and the effects of antidepressants, thereby demonstrating a promising antidepressant potential.

Genetic Val66Met BDNF Variant Increases Hyperphagia on Fat-rich Diets in Mice

High prevalence of obesity is attributable in part to consumption of highly palatable, fat-rich foods. However, the mechanism controlling dietary fat intake is largely unknown. In this study we investigated the role of brain-derived neurotrophic factor (BDNF) in the control of dietary fat intake in a mouse model that mimics the common human Val-to-Met (Val66Met) polymorphism that impairs BDNF release via the regulated secretory pathway. BdnfMet/Met mice gained weight much faster than wild-type (WT) mice and developed severe obesity due to marked hyperphagia when they were fed HFD. Hyperphagia in these mice worsened when the fat content in their diet was increased. Conversely, mice lacking leptin exhibited similar hyperphagia on chow and HFD. When 2 diets were provided simultaneously, WT and BdnfMet/Met mice showed a comparable preference for the more palatable diet rich in either fat or sucrose, indicating that increased hyperphagia on fat-rich diets in BdnfMet/Met mice is not due to enhanced hedonic drive. In support of this interpretation, WT and BdnfMet/Met mice increased calorie intake to a similar extent during the first day after chow was switched to HFD; however, WT mice decreased HFD intake faster than BdnfMet/Met mice in subsequent days. Furthermore, we found that refeeding after fasting or nocturnal feeding with HFD activated TrkB more strongly than with chow in the hypothalamus of WT mice, whereas TrkB activation under these 2 conditions was greatly attenuated in BdnfMet/Met mice. These results indicate that satiety factors generated during HFD feeding induce BDNF release to suppress excess dietary fat intake.

CRTC1 deficiency, specifically in melanocortin-4 receptor-expressing cells, induces hyperphagia, obesity, and insulin resistance

Melanocortin-4 receptor (MC4R) is a critical regulator of appetite and energy expenditure in rodents and humans. MC4R deficiency causes hyperphagia, reduced energy expenditure, and impaired glucose metabolism. Ligand binding to MC4R activates adenylyl cyclase, resulting in increased levels of intracellular cyclic adenosine monophosphate (cAMP), a secondary messenger that regulates several cellular processes. Cyclic adenosine monophosphate responsive element-binding protein-1-regulated transcription coactivator-1 (CRTC1) is a cytoplasmic coactivator that translocates to the nucleus in response to cAMP and is reportedly involved in obesity. However, the precise mechanism through which CRTC1 regulates energy metabolism remains unknown. Additionally, there are no reports linking CRTC1 and MC4R, although both CRTC1 and MC4R are known to be involved in obesity. Here, we demonstrate that mice lacking CRTC1, specifically in MC4R cells, are sensitive to high-fat diet (HFD)-induced obesity and exhibit hyperphagia and increased body weight gain. Moreover, the loss of CRTC1 in MC4R cells impairs glucose metabolism. MC4R-expressing cell-specific CRTC1 knockout mice did not show changes in body weight gain, food intake, or glucose metabolism when fed a normal-chow diet. Thus, CRTC1 expression in MC4R cells is required for metabolic adaptation to HFD with respect to appetite regulation. Our results revealed an important protective role of CRTC1 in MC4R cells against dietary adaptation.

Stimulation of Gs signaling in MC4R cells by DREADD increases energy expenditure, suppresses food intake, and increases locomotor activity in mice

The melanocortin 4 receptor (MC4R) plays an important role in the regulation of appetite and energy expenditure in humans and rodents. Impairment of MC4R signaling causes severe obesity. MC4R mainly couples to the G-protein G_s. Ligand binding to MC4R activates adenylyl cyclase resulting in increased intracellular cAMP levels. cAMP acts as a secondary messenger, regulating various cellular processes. MC4R can also couple with G_q and other signaling pathways. Therefore, the contribution of MC4R/G_s signaling to energy metabolism and appetite remains unclear. To study the effect of G_s signaling activation in MC4R cells on whole body energy metabolism and appetite, we generated a novel mouse strain that expresses a G_s-coupled designer receptors exclusively activated by designer drugs [Gs-DREADD (GsD)] selectively in MC4R-expressing cells (GsD-MC4R mice). Chemogenetic activation of the GsD by a designer drug [deschloroclozapine (DCZ); 0.01∼0.1 mg/kg body wt] in MC4R-expressing cells significantly increased oxygen consumption and locomotor activity. In addition, GsD activation significantly reduced the respiratory exchange ratio, promoting fatty acid oxidation, but did not affect core (rectal) temperature. A low dose of DCZ (0.01 mg/kg body wt) did not suppress food intake, but a high dose of DCZ (0.1 mg/kg body wt) suppressed food intake in MC4R-GsD mice, although either DCZ dose (0.01 or 0.1 mg/kg body wt) did not affect food intake in the control mice. In conclusion, the current study demonstrated that the stimulation of G_s signaling in MC4R-expressing cells increases energy expenditure and locomotor activity and suppresses appetite.NEW & NOTEWORTHY We report that G_s signaling in melanocortin 4 receptor (MC4R)-expressing cells regulates energy expenditure, appetite, and locomotor activity. These findings shed light on the mechanism underlying the regulation of energy metabolism and locomotor activity by MC4R/cAMP signaling.

Identification of the interactions between specific genetic polymorphisms and nutrient intake associated with general and abdominal obesity in middle-aged adults

BACKGROUND & AIMS

Comprehensive understanding of gene-diet interactions is necessary to establish proper dietary guidelines to prevent and manage general and abdominal obesity. We investigated the role of genetic variants and their interactions with general and abdominal obesity-associated nutrients using a largescale genome-wide association study of Korean adults.

METHODS

A total of 50,808 participants from a Korean genome and epidemiology study were included. Dietary intake was assessed using a food frequency questionnaire. Obesity was defined as a body mass index ≥25 kg/m². Abdominal obesity (AO) was defined as waist circumference ≥90 cm and 80 cm in males and females, respectively. Dietary nutrient intake was classified based on Korean Dietary Reference Intakes (DRIs). Odds ratios and 95% confidence intervals were calculated after adjusting for age, sex, exercise, smoking, alcohol drinking, total energy consumption, PC1, and PC2.

RESULTS

Among the individuals consuming fat (%) above DRI, carriers of Ca binding protein 39 (CAB39)- rs6722579 minor allele (A) have a higher risk of AO than those not carrying the SNP (odds ration [OR] = 3.73, p-value = 2.05e-07; interaction p-value = 1.80e-07). Among the individuals consuming vitamin C above DRI, carriers of carboxypeptidase Q (CPQ)- rs59465035 minor allele (T) have a lower risk of AO than those without that SNP (OR = 0.89, p-value = 1.44e-08; interaction p-value = 9.50e-06). The genetic association with obesity was stronger among individuals with a genetic variant rs4130113 near GHR gene region in those consume folate above DRI and with a genetic variant rs5760920 near CRYBB2 gene region in those consume vitamin B2 above DRI.

CONCLUSION

Our study results suggested that interactions of specific polymorphisms at loci and certain nutrients may influence obesity and abdominal obesity.

Deletion of the Feeding-Induced Hepatokine TSK Ameliorates the Melanocortin Obesity Syndrome

Work in recent decades has established that metabolic hormones released by endocrine cells and diverse other cell types serve to regulate nutrient intake and energy homeostasis. Tsukushi (TSK) is a leucine-rich repeat-containing protein secreted primarily by the liver that exerts an inhibitory effect on brown fat sympathetic innervation and thermogenesis. Despite this, physiological regulation of TSK and the mechanisms underlying its effects on energy balance remain poorly understood. Here we show that hepatic expression and plasma concentrations of TSK are induced by feeding and regulated by melanocortin-4 receptor (MC4R) signaling. We generated TSK and MC4R-double-knockout mice to elucidate the nature of cross talk between TSK and the central regulatory circuit of energy balance. Remarkably, TSK inactivation restores energy balance, ameliorates hyperphagia, and improves metabolic health in MC4R-deficient mice. TSK ablation enhances thermogenic gene expression in brown fat, dampens obesity-association inflammation in the liver and adipose tissue, and protects MC4R-null mice from diet-induced nonalcoholic steatohepatitis. At the cellular level, TSK deficiency augments feeding-induced c-Fos expression in the paraventricular nucleus of the hypothalamus. These results illustrate physiological cross talk between TSK and the central regulatory circuit in maintaining energy balance and metabolic homeostasis.

Identification of novel GPCR partners of the central melanocortin signaling

OBJECTIVE

Homo- or heterodimerization of G protein-coupled receptors (GPCRs) generally affects the normal functioning of these receptors and mediates the responses to a variety of physiological stimuli in vivo. It is well known that melanocortin-3 receptor (MC3R) and melanocortin-4 receptor (MC4R) are key regulators of appetite and energy homeostasis in the central nervous system. However, the GPCR partners of MC3R and MC4R are not well understood. Our objective is to analyze single cell RNA-seq datasets of the hypothalamus to explore and identify novel GPCR partners of MC3R and MC4R and examine the pharmacological effect on the downstream signal transduction and membrane translocation of melanocortin receptors.

METHODS

We conducted an integrative analysis of multiple single cell RNA-seq datasets to reveal the expression pattern and correlation of GPCR families in the mouse hypothalamus. The emerging GPCRs with important metabolic functions were selected for cloning and co-immunoprecipitation validation. The positive GPCR partners were then tested for the pharmacological activation, competitive binding assay and surface translocation ELISA experiments.

RESULTS

Based on the expression pattern of GPCRs and their function enrichment results, we narrowed down the range of potential GPCR interaction with MC3R and MC4R for further confirmation. Co-immunoprecipitation assay verified 23 and 32 novel GPCR partners that interacted with MC3R and MC4R in vitro. The presence of these GPCR partners exhibited different effects in the physiological regulation and signal transduction of MC3R and MC4R.

CONCLUSIONS

This work represented the first large-scale screen for the functional GPCR complex of central melanocortin receptors and defined a composite metabolic regulatory GPCR network of the hypothalamic nucleuses.

Monogenic human obesity syndromes

Neural circuits in the hypothalamus play a key role in the regulation of human energy homeostasis. A critical circuit involves leptin-responsive neurons in the hypothalamic arcuate nucleus (the infundibular nucleus in humans) expressing the appetite-suppressing neuropeptide proopiomelanocortin (POMC) and the appetite-stimulating Agouti-related peptide. In the fed state, the POMC-derived melanocortin peptide α-melanocyte-stimulating hormone stimulates melanocortin-4 receptors (MC4Rs) expressed on second-order neurons in the paraventricular nucleus of the hypothalamus (PVN). Agonism of MC4R leads to reduced food intake and increased energy expenditure. Disruption of this hypothalamic circuit by inherited mutations in the genes encoding leptin, the leptin receptor, POMC, and MC4R can lead to severe obesity in humans. The characterization of these and closely related genetic obesity syndromes has informed our understanding of the neural pathways by which leptin regulates energy balance, neuroendocrine function, and the autonomic nervous system. A broader understanding of these neural and molecular mechanisms has paved the way for effective mechanism-based therapies for patients whose severe obesity is driven by disruption of these pathways.

VIPergic neurons of the infralimbic and prelimbic cortices control palatable food intake through separate cognitive pathways

The prefrontal cortex controls food reward seeking and ingestion, playing important roles in directing attention, regulating motivation towards reward pursuit, and the assignment of reward salience and value. The cell types that mediate these behavioral functions, however, are not well described. We report here that optogenetic activation of vasoactive peptide expressing (VIP) interneurons in both the infralimbic (IL) and prelimbic (PL) divisions of the medial prefrontal cortex in mice is sufficient to reduce acute, binge-like intake of high calorie palatable food in the absence of any effect on low calorie rodent chow intake in the sated animal. In addition, we discovered that the behavioral mechanisms associated with these changes in feeding differed between animals that underwent either IL or PL VIPergic stimulation. While IL VIP neurons showed the ability to reduce palatable food intake, this effect was dependent upon the novelty and relative value of the food source. In addition, IL VIP neuron activation significantly reduced novel object- and novel social investigative behavior. Activation of PL VIP neurons, however, produced a reduction in high calorie palatable food intake that was independent of food novelty. Neither IL nor PL VIP excitation changed motivation to obtain food reward. Our data show how neurochemically-defined populations of cortical interneurons can regulate specific aspects of food reward-driven behavior, resulting in a selective reduction in intake of highly valued food.

Genetic variants help define the role of the MC4R C-terminus in signaling and cell surface stability

Screening 92,445 subjects in the Geisinger-Regeneron DiscovEHR cohort, we identified 5 patients heterozygous for nonsense mutations causing early terminations at Glu307 or Leu328 on the C-terminus of melanocortin 4 receptor (MC4R). Two Q307Ter carriers are severely obese (BMI > 40), while one is overweight (BMI > 25). One L328Ter carrier is overweight and the other is lean. Pedigree analysis for two Q307Ter carriers shows segregation of the variant with higher BMI. Functionally, MC4R(Q307Ter) eliminated receptor surface expression and signaling, while MC4R(L328Ter) functioned like the wild-type receptor. MC4R(Q307Ter) is therefore a loss of function (LOF) variant and the region between the two truncation sites identified in our patients is critical to MC4R function. Truncating MC4R at various C-terminal positions between these two variant sites, we find that cysteine318 sits at a critical junction for receptor trafficking and function. We show that MC4R is lipid modified at cysteine318 and cysteine319. Therefore, truncation early in the MC4R C-terminus results in haploinsufficiency in humans while truncation after the first lipid-modification site is well tolerated. MC4R haploinsufficiency clearly segregates with higher BMI; however, severe obesity is not fully penetrant even in MC4R LOF carriers, suggesting critical roles for environmental and lifestyle factors in MC4R monogenic obesity.

The Role of the Melanocortin System in Metabolic Disease: New Developments and Advances

Obesity is increasing in prevalence across all sectors of society, and with it a constellation of associated ailments including hypertension, type 2 diabetes, and eating disorders. The melanocortin system is a critical neural system underlying the control of body weight and other functions. Deficits in the melanocortin system may promote or exacerbate the comorbidities of obesity. This system has therefore generated great interest as a potential target for treatment of obesity. However, drugs targeting melanocortin receptors are plagued by problematic side effects, including undesirable increases in sympathetic nervous system activity, heart rate, and blood pressure. Circumnavigating this roadblock will require a clearer picture of the precise neural circuits that mediate the functions of melanocortins. Recent, novel experimental approaches have significantly advanced our understanding of these pathways. We here review the latest advances in our understanding of the role of melanocortins in food intake, reward pathways, blood pressure, glucose control, and energy expenditure. The evidence suggests that downstream melanocortin-responsive circuits responsible for different physiological actions do diverge. Ultimately, a more complete understanding of melanocortin pathways and their myriad roles should allow treatments tailored to the mix of metabolic disorders in the individual patient.

Mechanisms and significance of brain glucose signaling in energy balance, glucose homeostasis, and food-induced reward

The concept that hypothalamic glucose signaling plays an important role in regulating energy balance, e.g., as instantiated in the so-called "glucostat" hypothesis, is one of the oldest in the field of metabolism. However the mechanisms by which neurons in the hypothalamus sense glucose, and the function of glucose signaling in the brain, has been difficult to establish. Nevertheless recent studies probing mechanisms of glucose signaling have also strongly supported a role for glucose signaling in regulating energy balance, glucose homeostasis, and food-induced reward.

Divergent effects of central melanocortin signalling on fat and sucrose preference in humans

Melanocortin-4-receptor (MC4R)-expressing neurons modulate food intake and preference in rodents but their role in human food preference is unknown. Here we show that compared with lean and weight-matched controls, MC4R deficient individuals exhibited a markedly increased preference for high fat, but a significantly reduced preference for high sucrose food. These effects mirror those in Mc4r null rodents and provide evidence for a central molecular circuit influencing human macronutrient preference.

Hypothalamic melanocortin-4-receptors (MC4R) regulate food preference in rodents, but their role in humans is unclear. Here, the authors perform food preference and liking tests in humans with MC4R mutations and find that they prefer fatty food more, but sweet food less, than people without MC4R mutations.

Inhibitory Effect of the Melanocortin Receptor Agonist Melanotan-II (MTII) on Feeding Depends on Dietary Fat Content and not Obesity in Rats on Free-Choice Diets

Introduction: Conflicting data exist on sensitivity changes of the melanocortin system during diet-induced obesity. We hypothesized that melanocortin sensitivity depends on diet composition, in particular on the fat content rather than the level of obesity. The aim of this study was to determine the influence of diet composition on feeding responses to a melanocortin receptor agonist, using free-choice diets that differ in food components.

Methods: Male Wistar rats were subjected to a chow (CHOW) diet or a free-choice (fc) diet of either chow, saturated fat and liquid sugar (fcHFHS), chow and saturated fat (fcHF), or chow and liquid sugar (fcHS) for 4 weeks. Melanocortin sensitivity was tested by measuring food intake following administration of the melanocortin 3/4 receptor agonist melanotan II (MTII) or vehicle in the lateral ventricle. In a separate experiment, proopiomelanocortin (POMC) and agouti-related protein (AgRP) mRNA levels were determined in the arcuate nucleus with in situ hybridization in rats subjected to the free-choice diets for 4 weeks.

Results: Rats on the fcHFHS diet for 4 weeks show increased caloric intake and body weight gain compared to rats on the CHOW, fcHS and fcHF diet. Caloric intake and body weight gain was comparable between rats on the fcHF, fcHS, and CHOW diet. After 4 weeks diet, POMC and AgRP mRNA levels were not different between diet groups. MTII inhibited caloric intake to a larger extent in rats on the fcHF diet compared to rats on the CHOW, fcHFHS or fcHS diet. Moreover, the fat component was the most inhibited by MTII, and the sugar component the least.

Conclusion: Rats on the fcHF diet show stronger food intake inhibition to the melanocortin receptor agonist MTII than rats on the CHOW, fcHS, and fcHFHS diet, which is independent of caloric intake and body weight gain. Our data point toward an important role for diet composition, particularly the dietary fat content, and not obesity in the sensitivity of the melanocortin system.

Moderate voluntary exercise attenuates the metabolic syndrome in melanocortin-4 receptor-deficient rats showing central dopaminergic dysregulation

Objective

Melanocortin-4 receptors (MC4Rs) are highly expressed by dopamine-secreting neurons of the mesolimbic tract, but their functional role has not been fully resolved. Voluntary wheel running (VWR) induces adaptations in the mesolimbic dopamine system and has a myriad of long-term beneficial effects on health. In the present experiments we asked whether MC4R function regulates the effects of VWR, and whether VWR ameliorates MC4R-associated symptoms of the metabolic syndrome.

Methods

Electrically evoked dopamine release was measured in slice preparations from sedentary wild-type and MC4R-deficient Mc4r^K314X (HOM) rats. VWR was assessed in wild-type and HOM rats, and in MC4R-deficient loxTB^Mc4r mice, wild-type mice body weight-matched to loxTB^Mc4r mice, and wild-type mice with intracerebroventricular administration of the MC4R antagonist SHU9119. Mesolimbic dopamine system function (gene/protein expression) and metabolic parameters were examined in wheel-running and sedentary wild-type and HOM rats.

Results

Sedentary obese HOM rats had increased electrically evoked dopamine release in several ventral tegmental area (VTA) projection sites compared to wild-type controls. MC4R loss-of-function decreased VWR, and this was partially independent of body weight. HOM wheel-runners had attenuated markers of intracellular D1-type dopamine receptor signaling despite increased dopamine flux in the VTA. VWR increased and decreased ΔFosB levels in the nucleus accumbens (NAc) of wild-type and HOM runners, respectively. VWR improved metabolic parameters in wild-type wheel-runners. Finally, moderate voluntary exercise corrected many aspects of the metabolic syndrome in HOM runners.

Conclusions

Central dopamine dysregulation during VWR reinforces the link between MC4R function and molecular and behavioral responding to rewards. The data also suggest that exercise can be a successful lifestyle intervention in MC4R-haploinsufficient individuals despite reduced positive reinforcement during exercise training.

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MC4R-deficiency causes metabolic syndrome.

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Loss of MC4R signaling decreases voluntary wheel running (VWR).

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Despite moderate amounts of VWR, MC4R-associated metabolic syndrome is severely attenuated.

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MC4R-deficiency is associated with mesolimbic dopamine dysregulation during VWR.

The hunger genes: pathways to obesity

The global rise in the prevalence of obesity and associated co-morbidities such as type 2 diabetes, cardiovascular disease, and cancer represents a major public health concern. The biological response to increased consumption of palatable foods or a reduction in energy expenditure is highly variable between individuals. A more detailed mechanistic understanding of the molecular, physiological, and behavioral pathways involved in the development of obesity in susceptible individuals is critical for identifying effective mechanism-based preventative and therapeutic interventions.

Mitigating or exacerbating effects of maternal-fetal programming of female mice through the food choice environment

Humans live, eat, and become overweight/obese in complex surroundings where there are many available food choices. Prenatal exposure to poor food choices predisposes offspring to increased negative health risks, including obesity. Many animal experiments have analyzed intergenerational body weight parameters in an environment without food choices, which may not be directly translatable to the human food environment. In this study, offspring from mothers with a defined high-fat diet (HFD) or low-fat diet (LFD) were arbitrarily assigned to either an exclusively LFD or HFD or to a diet where they have a choice between LFD and HFD (choice diet). Offspring displayed negative outcomes of increased body weight, body fat, serum leptin, and blood glucose levels when given the choice diet compared with offspring on the LFD. Conversely, improved energy expenditure was found for offspring given the choice diet compared with offspring from HFD dams given LFD. In addition, maternal diet-specific influences on offspring metabolic parameters were identified, especially in offspring from HFD dams, including positive outcomes of reduced leptin in LFD offspring, reduced corticosterone and cholesterol levels in HFD offspring, and increased exercise levels in choice offspring, as well as the negative outcome of increased calorie intake in LFD offspring from HFD dams. This defined model can now be used as the basis for future studies to characterize the cycle of inter- and intragenerational obesity and whether more realistic diet environments, especially those including choice, can mitigate phenotype.

Association between MC4R rs17782313 polymorphism and overeating behaviors

BACKGROUND/OBJECTIVES

Melanocortins have a crucial role in appetite and weight regulation. Although the melanocortin 4 receptor (MC4R) gene has been repeatedly linked to obesity and antipsychotic-induced weight gain, the mechanism behind how it leads to this effect in still undetermined. The goal of this study was to conduct an in-depth and sophisticated analysis of MC4R polymorphisms, body mass index (BMI), eating behavior and depressed mood.

SUBJECTS/METHODS

We genotyped 328 individuals of European ancestry on the following MC4R markers based on the relevant literature on obesity and antipsychotic-induced weight gain: rs571312, rs17782313, rs489693, rs11872992, and rs8087522. Height and weight were measured, and information on depressed mood and overeating behaviors was obtained during the in-person assessment.

RESULTS

BMI was associated with rs17782313 C allele; however, this finding did not survive correction for multiple testing (P = 0.018). Although rs17782313 was significantly associated with depressed mood and overeating behaviors, tests of indirect effects indicated that emotional eating and food cravings, rather than depressed mood, uniquely accounted for the effect of this marker and BMI (n = 152).

CONCLUSIONS

To our knowledge, this is the first study to investigate the link between MC4R rs17782313, mood and overeating behavior, as well as to demonstrate possible mechanisms behind MC4R's influence on body weight. If replicated in a larger sample, these results may have important clinical implications, including potential for the use of MC4R agonists in the treatment of obesity and disordered eating.

The melanocortin-4 receptor is expressed in enteroendocrine L cells and regulates the release of peptide YY and glucagon-like peptide 1 in vivo

The melanocortin-4 receptor (MC4R) is expressed in the brainstem and vagal afferent nerves and regulates a number of aspects of gastrointestinal function. Here we show that the receptor is also diffusely expressed in cells of the gastrointestinal system, from stomach to descending colon. Furthermore, MC4R is the second most highly enriched GPCR in peptide YY (PYY) and glucagon-like peptide 1 (GLP-1) expressing enteroendocrine L cells. When vectorial ion transport is measured across mouse or human intestinal mucosa, administration of α-MSH induces a MC4R-specific PYY-dependent antisecretory response consistent with a role for the MC4R in paracrine inhibition of electrolyte secretion. Finally, MC4R-dependent acute PYY and GLP-1 release from L cells can be stimulated in vivo by intraperitoneal (i.p.) administration of melanocortin peptides to mice. This suggests physiological significance for MC4R in L cells and indicates a previously unrecognized peripheral role for the MC4R, complementing vagal and central receptor functions.

Elimination of Kalrn expression in POMC cells reduces anxiety-like behavior and contextual fear learning

Kalirin, a Rho GDP/GTP exchange factor for Rac1 and RhoG, is known to play an essential role in the formation and maintenance of excitatory synapses and in the secretion of neuropeptides. Mice unable to express any of the isoforms of Kalrn in cells that produce POMC at any time during development (POMC cells) exhibited reduced anxiety-like behavior and reduced acquisition of passive avoidance behavior, along with sex-specific alteration in the corticosterone response to restraint stress. Strikingly, lack of Kalrn expression in POMC cells closely mimicked the effects of global Kalrn knockout on anxiety-like behavior and passive avoidance conditioning without causing the other deficits noted in Kalrn knockout mice. Our data suggest that deficits in excitatory inputs onto POMC neurons are responsible for the behavioral phenotypes observed.

Association between melanocortin-4 receptor mutations and eating behaviors in obese patients: a case--control study

Melanocortin-4 receptor (MC4R) gene mutations are involved in the leptin-melanocortin pathways that control food intake. The effect of these mutations on eating behavior phenotypes is still debated. To determine the association between functional MC4R mutations and eating behaviors, dietary intake and physical activity, we sequenced the MC4R gene in 4653 obese adults. Among them, 19 adults carriers of functional MC4R mutation were matched on age, sex and body mass index with two randomly-paired controls without MC4R mutation (n=57). We found that eating behaviors and physical activity did not differ between groups. In particular, cases were not at increased risk of binge eating disorders. Subjects carriers of MC4R mutation reported a higher proportion of dietary carbohydrates intakes (43.2±7.1 and 39.2±8.1% of total energy intake, respectively, P=0.048) and a lower proportion of dietary lipids (34.3±6.7 and 38.5±6.7% of total energy intake, respectively, P=0.018). In conclusion, mutation carriers differ from controls by a higher consumption of carbohydrates counterbalanced by a lower consumption of lipids expressed as percentage of total energy intake. However, functional MC4R mutations do not have a higher risk of compulsive eating contrary to what was previously suggested.

The expression of MC4Rs in D1R neurons regulates food intake and locomotor sensitization to cocaine

While it is known that mice lacking melanocortin 4 receptor (MC4R) expression develop hyperphagia resulting in early-onset obesity, the specific neural circuits that mediate this process remain unclear. Here, we report that selective restoration of MC4R expression within dopamine-1 receptor-expressing neurons [MC4R/dopamine 1 receptor (D1R) mice] partially blunts the severe obesity seen in MC4R-null mice by decreasing meal size, but not meal frequency, in the dark cycle. We also report that both acute cocaine-induced anorexia and the development of locomotor sensitization to repeated administration of cocaine are blunted in MC4R-null mice and normalized in MC4R/D1R mice. Neuronal retrograde tracing identifies the lateral hypothalamic area as the primary target of MC4R-expressing neurons in the nucleus accumbens. Biochemical studies in the ventral striatum show that phosphorylation of DARPP-32(Thr) (-34) and GluR1(Ser) (-845) is diminished in MC4R-null mice after chronic cocaine administration but rescued in MC4R/D1R mice. These findings highlight a physiological role of MC4R-mediated signaling within D1R neurons in the long-term regulation of energy balance and behavioral responses to cocaine.